1. Field of the Invention
This invention pertains to inspection of plastic joints and more particularly to a non-destructive plastic butt-weld joint inspection method with emphasis on plastic pipe butt-weld joints.
2. Background of the Invention
Butt fusion processes have been successfully used for joining plastic pipes in water and gas-distribution systems for nearly three decades. As in many other critical applications, the quality of the joints greatly effects the overall operational safety of, the system. While mar failures of pipe butt fusion joints are infrequent, when they do occur, they can be dangerous and can result in significant loss.
Typically, butt joints fall because of weakened or incomplete fusion of the mating surfaces. This can occur as a result of several factors. For example, joint failure can occur as a result of insufficient pressure at portions of or over the entire periphery of the joint. Failure can occur due to premature solidification of the molten material in the weld zone due to weather conditions or due to an overextended open time. That is, the molten ends of the surfaces to be joined are allowed to cool excessively prior to the application of a bonding force. Failure can also occur due to contamination of the molten plastic
Although ultrasonic instruments are available for nondestructive inspection of butt fusion joints, these devices are very expensive and their effectiveness and reliability for field inspection has not been fully ascertained. As a result, such instruments are not widely used. While it is possible to make cross-sectional cuts through the weld and make visual inspection using heating techniques to bring out joint defects, such destructive testing is useful only for training purposes. Clearly the destruction of the joint is counterproductive in a working system.
Presently, there is no simple and reliable method available for the non-destructive inspection of butt fusion joints. Typically the joints are assessed by visual inspection based on the size and shape of the weld bead. However, this method is subjective and can be misleading. For example, with an excessive, joint-formation heating time, most of the pipe material is displaced into the bead during the heating cycle rather than as a result of the joining (forging) pressure. Although visual inspection suggests an acceptable bead appearance, the joint is defective because of the excess loss of plastic in the joint region to the weld. As such, visual inspection simply cannot provide direct information as to the mechanical properties of a joint that result from the fusion process. The bead itself, plays an insignificant role in the joint strength and thus is not a reliable indicator of the underlying joint.
To overcome these problems, it is an object of the present invention to provide a non-destructive method of observing the quality of plastic butt joints.
Another object of the present invention is to provide an inexpensive method for the field testing of plastic joints such as plastic pipe joints.
A further object of the present invention is to provide a permanent record of the weld joint that can be stored for future reference.
Another object of the present invention is to enhance the accuracy of in-field weld inspection.
Yet another object of the present invention is to provide repeatable weld joint test results.